The present invention relates generally to a component within a control system, and more particularly to a programmable current loop interface that supports Highway Adressable Remote Transfer (HART) protocol signals with point galvanic isolation, for use in controlling and monitoring a HART device.
Control of a steam turbine or gas turbine is one example in which a HART device has industrial applicability. A HART device is generally a sensor or actuator that is embedded with the “smarts” to facilitate communication with a control device using the well-known HART protocol. In the control of a turbine, a HART sensor device controls current flowing in a wire, while a HART actuator device is controlled by the flow of the current in the wire. In accordance with the HART protocol, these functions are performed over the current loop carrying a 4 to 20 milliampere (mA) signal that is superimposed with HART data tones.
In some turbine control systems, HART sensor devices and HART actuator devices may be interfaced to the control system using modules that perform current loop measurements and current loop driving. These HART device modules are generally implemented to have a specified ratio of input channels to output channels in order to perform the current loop measurement and current loop driving functions. Typically, the ratio of input channels to output channels is fixed. Often times, the amount of sensors and actuators used in the control of a turbine does not conform to the specified ratio of input channels to output channels provided in a typical module. For example, a 12 channel module having 10 inputs and 2 outputs may not be well suited for use in a control application that does not have a use for all of the inputs and outputs. For instance, if a control application is desired to have 96 outputs and zero inputs, then it would be necessary to use 48 packs of these 12 channel modules in order to attain the desired ratio of zero inputs to 96 outputs. This is an inefficient use of these modules because all of the inputs (i.e., 480) in the 48 packs would be inactive. In addition, using 48 packs of 12 channel modules to attain the desired ratio of zero inputs to 96 outputs makes the cost of implementing the control application expensive.